Engine harmonic cancellation systems are adaptive feed-forward noise reduction systems that are used in motor vehicles, for example in cabins or in muffler assemblies, to reduce or cancel engine harmonic noise. A sine wave at the frequency to be cancelled is used as an input to an adaptive filter. Engine harmonic cancellation systems also use one or more microphones as error input transducers. The adaptive filter can alter the magnitude and/or the phase of the input sine wave. The output of the adaptive filter is applied to one or more transducers that produce sound (i.e., loudspeakers) that is acoustically opposite to the undesirable engine harmonics that are to be canceled. The aim of the system is to cancel the noise at the frequency or frequencies of interest by adaptively minimizing the total energy across all error microphone input signals. In order to do so, the loudspeaker outputs have a negative gain.
Harmonic noise cancellation systems are also used to cancel or reduce noise caused by noise sources other than engines. One additional source of noise in motor vehicles is the propeller shaft, also known as the drive shaft. Because geared transmissions are used to transfer engine rotation to propeller shaft rotation, the propeller shaft rotation rate is not fixed relative to the engine rotation rate. The engine and propeller shaft thus can be sources of noise in a vehicle cabin at different frequencies.
In order to cancel noise from both an engine and a propeller shaft, a noise reduction system requires two feed-forward adaptive filters. When the two frequencies being cancelled are coincident or close, the stability margins of the filters can be compromised. This increases the possibility of divergence of the filter algorithms, which can lead to the creation of loud and noticeable noise artifacts.